1. Field of the Invention
The present invention relates to a transmission power control system of a mobile terminal in a mobile communication network and a method of transmission power control thereof. More particularly, the present invention concerns a transmission power control in which a quality of a radio signal transmitted from a mobile terminal under communication is measured in a radio base station and the measured result is reported to a radio base station control apparatus, and then in response to the measured quality of the radio signal, the radio base station control apparatus instructs the mobile terminal through the radio base station to change (increase or decrease) the transmission power thereof.
2. Description of the Related Art
In general, a mobile communication system is arranged by a plurality of radio base stations for constituting a plurality of radio zones, and a radio base station control apparatus, which entirely controls these plural radio base stations, connected to a mobile communication switching station interfaced with a fixed communication network. FIG. 8 is a flow chart for indicating one example of the conventional transmission power control method for the mobile terminal in such a mobile communication system. The radio base station control apparatus transfers the power control information to the radio base station in response to the radio signal quality of the mobile terminal measured by the radio base station, and the radio base station informs the power control information to the mobile terminal by using the down-stream signal channel. As indicated in FIG. 8, the power control information includes a power control bit to indicate change of power to be transmitted from the mobile terminal. When the power control bit=1, it indicates that the transmission output is to be increased by one step of value which has been predetermined, and, to the contrary, when the power control bit =0, it indicates that the transmission output is to be decreased by one step of value which has also been predetermined. In response to the respective instructions, the mobile terminal increases or decreases the transmission power with a predetermined changing width by 1 step.
Other than such a control method, there is a further method for autonomously controlling the transmission power based upon the reception field strength of the radio signal transmitted from the radio base station and received by this mobile terminal (see Japanese Laid-open Patent Application No. shou 61-43026). Also, in the method of instructing a change in the transmission power from the radio base station control apparatus, number of steps to be changed for increasing, or decreasing the transmission power is instructed.
In the above-described conventional transmission power control method for the mobile terminal, when the mobile terminal autonomously controls the transmission power, this power control is carried out based on the reception field strength at the mobile terminal. As a consequence, this power control will accept a temporal level down in the reception field strength due to such as the fading phenomenon. Therefore, the mobile terminal does not always output the optimum transmission power.
In another case that the transmission power is controlled in response to the instruction issued from the radio base station control apparatus, it will take a lengthy time period until the transmission power output value reaches the optimum value. There is such a problem that since the mobile terminal whose power supply has been just turned ON firstly transmits the radio signal in the maximum transmission power, waste power consumption occurs until this maximum transmission power reaches the optimum transmission power. There is another problem that when the radio signal is transmitted in high power by this mobile terminal, other mobile terminals will receive interference. Therefore, it is required to quickly control the transmission power to become the optimum transmission power within a short time.